My wife and I have cruised 15 times with RCI and two times each on Carnival and Princess. We found Princess to be too laid back. It is a much older crowd. It appeared that most people there had been in retirement for 25 plus years. Things closed down too early. DW and I are in our early Fifties.
Carnival on the other hand was a bit too live for us. We are not the "boogie down, party hardy" animals we use to be. We still like to "get down", we just do it with a little more ease. We found that Royal Caribbean is just the right speed for us. No knock against the other lines, we just prefere RCI. By the way you can check out RCI's Military rates. They do have some. Enjoy.